Recent improvement in multimedia technology has increased an amount of information to deal with, thereby demanding a memory device having a large capacity. It has been intensively studied how to improve a density in memory devices, especially a rewritable optical disc, a magnetic disc and a magnetic tape.
As one of such technologies how to improve the density of the memories, suggested is a recording-reproduction method that uses a magnetic recording medium having magnetic characteristics varied according to temperature, and locally heats up a portion of the magnetic recording medium, for example, by means of projection of light onto the portion (the portion locally heated is referred to as a heated area, hereinafter), and performs magnetic recording or reproduction of the heated area exclusively, thereby realizing recording and reproduction of a high density.
A method, which performs magnetic recording or reproduction of information in the locally-heated area of such a magnetic recording medium, is called a heat-assisting magnetic signal recording method or a heat-assisting magnetic signal reproduction method (a heat-assisting magnetic signal recording-reproduction method), for example. Especially, in case optical means is used for locally heating up the heated area, such a method is denoted as a light-assisting magnetic signal recording method or a light-assisting magnetic signal reproduction method (a light-assisting magnetic signal recording-reproduction method).
As examples of the heat-assisting magnetic signal recording-reproduction method, suggested are a variety of such methods in which (a) used as a recording medium is a vertical magnetic film made of a ferrimagnetic material having a magnetic compensation point temperature (that is, a temperature at which magnetization is zero) in a vicinity of room temperature, (b) during recording, a light beam is projected onto an area for recording in the recording medium so as to heat up the area for recording to a vicinity of Curie temperature, then an external magnetic field is applied into the area for recording by using a recording head so as to record information, (c) during reproduction, the light beam is projected onto an area for reproduction in the recording medium so as to heat up the area for reproduction in order to facilitate magnetization in the area for reproduction, then a magnetic flux leaked out of the area for reproduction is detected by a reproduction head so as to reproduce the information.
By the way, in the heat-assisting magnetic recording method, a shape of a magnetic bit depends on a temperature distribution and a magnetic field distribution.
For example, a converging laser beam, which is common means for locally heating up the heated area, produces a heat distribution in a circle or substantially ellipsoidal shape on the recording medium. Meanwhile, a ring head, which is commonly used as a recording magnetic head at present, makes a magnetic field distribution in a substantially rectangular shape. For this reason, the magnetic bit recorded has a shape which is resulted from a combination of both of the shapes.
However, a reproduction head, which is commonly used at present, has a substantially rectangular shape, thus its reproduction region is in the substantially rectangular shape. Therefore, the creation of the magnetic bit in a non-rectangular shape as described above causes such a problem that an S/N of a signal is adversely affected, thereby giving a drawback to achieve the recording-reproduction process having high bit density.
According to “published Japanese translation of PCT international publication for patent applications (Tokuhyohei) No. 6-500194 (published on Jan. 6, 1994) (hereinafter, referred to as the Reference 1), in which discussed is how to decide the shape of the magnetic bit in the heat-assisting magnetic recording method, the following conditions are effective to record a magnetic bit in a substantially rectangular shape:
(1) a coercive force of the magnetic recording medium is substantially constant along a direction of tracks,
(2) contour lines of the coercive force (hereinafter, just referred to as coercive force contour lines) of the magnetic recording medium are substantially parallel to the direction of the track,
(3) in order to attain the conditions (1) and (2), created on the magnetic recording medium is a temperature distribution having a shape long in the direction of the track.
(4) to attain the condition (3), used as the heating means is the converged laser beam that has a focal image in an oblong shape,
(5) to attain the condition (3), used is a magnetic recording medium having thermal anisotropy in the direction of the track, and
(6) the coercive force of the medium is constant in a vicinity of a recording temperature, regardless of the temperature.
However, the Reference 1 fails to take the magnetic field distribution of the magnetic recording head into consideration. The inventors of the present invention have found out that (a) the technology disclosed in the cited Reference 1 is not able to record a magnetic bit having a rectangular shape, which cannot be produced in the technology, and (b) the technology disclosed in the Reference is not suitable for the high bit density recording because the magnetic field distribution of the magnetic recording head is not taken into consideration, as specifically explained below.
As discussed above, the shape of the magnetic bit is determined by a change in the magnetic characteristics of the magnetic recording medium due to temperature, the temperature distribution formed on the magnetic recording medium, and the magnetic field applied onto the magnetic recording medium.
Here, it is assumed that the magnetic recording medium is a uniaxial magnetic anisotropic medium having an axis of easy magnetization (easy axis) along a vertical direction with respect to a surface of the magnetic recording medium. Because, in the magnetic recording medium, only a component, which is vertical to the surface of the magnetic recording medium, of the magnetic field contributes the recording in the magnetic recording medium. Therefore, the component of the magnetic field will be called as a vertical magnetic field intensity or a recording magnetic field intensity, hereinafter.
Note that, since only the component of the magnetic field vertical to the surface of the magnetic recording medium (film surface), a coercive force applied onto the magnetic field in the vertical direction to the surface of the magnetic recording medium is denoted as a coercive force of the magnetic recording medium, hereinafter.
In FIG. 17, shown is a relationship between the coercive force, which is applied on to the magnetic field in the vertical direction to the film surface, and the temperature in the magnetic recording medium.
Furthermore, it is assumed that the temperature distribution formed on the magnetic recording medium is in an oblong shape, as recited in the Reference 1, for convenience to explain.
Another assumption is the vertical magnetic field intensity applied onto the magnetic recording medium is constant in a direction of a track pitch, for convenience to explain. Thus, contour lines of the vertical magnetic field intensity (hereinafter, just referred to as magnetic field intensity contour lines) are lines vertical with respect to the direction of the track pitch.
FIG. 12 is a graph explaining how the vertical magnetic field intensity is distributed on the magnetic recording medium by a common ring head.
The above assumptions and the conditions give a temperature distribution and a magnetic field distribution, which are recommended in the Reference 1 so as to attain the magnetic bit having the rectangular shape. The temperature distribution and the magnetic field distribution are indicated by coercive force contour lines 701 and magnetic field intensity contour lines 601 in FIG. 18. Note that, in FIG. 18, illustrated is a positional relationship between the coercive force contour lines 701 and the magnetic field contour lines 601 on the magnetic recording medium, based on the technology disclosed in Reference 1.
Here, the heat-assisting magnetic recording is carried out in a region in which the coercive force of the magnetic recording medium falls below a recording magnetic field intensity. The region is highlighted by using crosshatching in FIG. 18, and is called as a recordable region 800. Note that, only a trailing edge of the recordable region 800 concerns to the determination of the shape of the magnetic bit. This will be explained later. Therefore, hereinafter, an explanation on a part of the recordable region, which does not contribute to the determination of the shape of the magnetic bit, will be omitted.
The magnetic bit is formed by magnetizing the magnetic recording medium in different directions in the vertical direction with respect to the film surface, while moving the magnetic recording medium. The formation of the magnetic bit is illustrated in FIGS. 19(a) and 19(b). A virtual mark 901 in FIGS. 19(a) and 19(b) is a virtual (imaginary) point fixed on the magnetic recording medium in order to assist the understanding of movement of the magnetic recording medium. It is explicit here that the magnetic bit recorded in such a shape that its edges in the track direction is shaped by a shape of the trailing edge of the recordable region in the track direction.
In short, the shape of the trailing edge of the recordable region 800 determines the shape of the edges of the magnetic bit in the track direction. Hereinafter, the trailing edge of the recordable is called a recording edge 801.
As described above, the shape of the magnetic bit is dependant on the shape of a recording edge that is determined according to the coercive force distribution in the direction vertical to the track and the magnetic field intensity in the track direction. Thus, in order to record the magnetic bit in the rectangular shape, it is preferable that the recording edge has a shape approximate to a linear shape that is vertical with respect to the track direction.
Therefore, similar to the case where the technology of the Reference 1 is employed, even though the temperature distribution having the oblong shape is assumed, the recording edge is a curve, thereby giving the magnetic bit a curved section having a width in correspondence with the magnetic field distribution.
Therefore, it is impossible to form the magnetic bit having the substantially rectangular shape by using the temperature distribution having the oblong shape recited in the Reference 1.
Moreover, a common magnetic recording-reproduction apparatus uses a magnetic recording medium having a disc shape, so that various different addresses can be easily accessed. Thus, the common magnetic recording-reproduction apparatus is provided with means for revolving the magnetic recording medium having the disc shape, and means for moving a recording-reproduction head in a substantially radial direction of the disc-shaped magnetic recording medium.
In many cases, because the means for moving the recording-reproduction head is rotated, the recording-reproduction head makes a difference angle with a moving direction of the magnetic recording medium, depending on where the recording-reproduction head has an access position. In other words, the access position of the recording-reproduction head changes an angle between the magnetic field distribution and the temperature distribution, thereby changing the shape of the recording edge. FIG. 20 illustrates how the shape of the recording edge is changed. The change is more enlarged as the temperature distribution to apply is longer in the track direction. Thus, it is proved that the temperature distribution having the oblong shape recited in the Reference 1 is not adequate to be used in the high density recording.
In addition, the formation of the temperature distribution having the oblong shape heats up a large area of the magnetic recording medium, thereby having such a disadvantage that a large amount of electric power is consumed.
Furthermore, the use of the recording-reproduction head having the temperature distribution having the oblong shape maintains a temperature of the magnetic bit at a vicinity of a recording temperature for a significant period of time after the recording-reproduction head having the temperature distribution passes through a recording region. Thus, the magnetic bit may become instable due to this. Further, the magnetic field is disturbed, for example, by another magnetic field of another magnetic bit, thereby partially changing a position of the recording edge 801. As a result, the shape of the magnetic bit recorded may be altered in a long area. This means that the shape of the magnetic bit recorded may not be constant even if a short magnetic bit is recorded for a sake of high density recording.
Moreover, while it is preferable that the magnetic bit is ended by reduction of the coercive force in accordance with the temperature distribution so as to make the best of a high track density of the optical recording. However, the statement that the coercive force is constant in the vicinity of the recording temperature is not enough to explain the feature. That is, in the medium recited in the Reference 1, the track pitch is determined mainly by a width of a recording gap.
Therefore, in the conventional light-assisting magnetic recording method, with the method recited in the Reference 1, the magnetic bit having the rectangular shape can be recorded in a substantially constant shape only if the magnetic bit has a longer length than the gap of the magnetic recording head. Thus, the method of the Reference 1 is inadequate for the high density recording and reproduction, in terms of the S/N.
Moreover, in a case where the common disc drive is used to realize the method recited in the Reference 1, it is necessary to employ a signal processing method in accordance with the magnetic bit that is varied according to an access position of the magnetic recording head.
Furthermore, in the method of the Reference 1, extra power is required for a laser used for the recording. Moreover, the magnetic bit recorded is very instable against heat.
Moreover, in case where the magnetic recording medium recited in the Reference 1 is used in the common disc drive, the track pitch is determined by the width of the gap of the magnetic recording head, so that it is impossible to make the best of the high track density of the optical recording.